1. Field of the Invention
This invention relates to nuclear magnetic resonance and more particularly to a nuclear magnetic resonance apparatus and method using high static magnetic fields trapped in high temperature superconducting materials.
2. Description of the Related Art
A variety of techniques have are utilized in determining the presence and estimation of quantities of hydrocarbons (oil and gas) in earth formations. These methods are designed to determine formation parameters, including among other things, the resistivity, porosity and permeability of the rock formation surrounding the wellbore drilled for recovering the hydrocarbons. Typically, tools designed to provide the desired information are used to log the wellbore. Much of the logging is done after the well bores have been drilled. More recently, wellbores have been logged while drilling of the wellbores, which is referred to as measurement-while-drilling (xe2x80x9cMWDxe2x80x9d) or logging-while-drilling (xe2x80x9cLWDxe2x80x9d).
When logging is done after the wellbores have been drilled, a sensor assembly is conveyed downhole on a wireline that includes electrically conducting cables for carrying electrical power downhole and for transmission of signals in an uphole and a downhole direction.
In MWD applications, a drilling assembly (also referred to as the xe2x80x9cbottom hole assemblyxe2x80x9d or the xe2x80x9cBHAxe2x80x9d) carrying a drill bit at its bottom end is conveyed into the wellbore or borehole. The drilling assembly is usually conveyed into the wellbore by a coiled-tubing or a drill pipe. In the case of the coiled-tubing, the drill bit is rotated by a drilling motor or xe2x80x9cmud motorxe2x80x9d which provides rotational force when a drilling fluid is pumped from the surface into the coiled-tubing. In the case of the drill pipe, it is rotated by a power source (usually an electric motor) at the surface, which rotates the drill pipe and thus the drill bit.
Bottom hole assemblies generally include several formation evaluation sensors for determining various parameters of the formation surrounding the BHA during the drilling of the wellbore. Such sensors are usually referred to as the MWD sensors. Such sensors traditionally have electromagnetic propagation sensors for measuring the resistivity, dielectric constant, water saturation of the formation, nuclear sensors for determining the porosity of the formation and acoustic sensors to determine the formation acoustic velocity and porosity. Other downhole sensors that have been used or proposed for use include sensors for determining the formation density and permeability. The bottom hole assemblies also include devices to determine the BHA inclination and azimuth, pressure sensors, temperature sensors, gamma ray devices, and devices that aid in orienting the drill bit in a particular direction and to change the drilling direction. Acoustic and resistivity devices have been proposed for determining bed boundaries around and in some cases in front of the drill bit. More recently, nuclear magnetic resonance (NMR) sensors have gained extreme interest as MWD sensors as such sensors can provide direct measurement for water saturation porosity and indirect measurements for permeability and other formation parameters of interest.
NMR tools generate a near uniform static magnetic field in a region of interest surrounding the wellbore. The NMR measurement is based on the fact that the nuclei of many elements possess angular momentum (xe2x80x9cspinxe2x80x9d) and a magnetic moment. In the absence of an external field, the nuclear spin orientations are randomly distributed with an essentially uniform orientation in space, but when a magnetic field is applied, the nuclei tend to align themselves in one of two quantum states: either parallel or anti-parallel to the applied field. There is a net excess of spins aligned parallel to the field, so that on a macroscopic level, the material in the region of interest takes on a net magnetization aligned in the same direction as the applied magnetic field. The stronger the magnetic field, the greater the excess of parallel spins and the stronger the net magnetization. NMR sensors utilize permanent magnets to generate a static magnetic field in the formation surrounding the MWD tool.
For the purposes of this invention, the NMR measurements may be treated on a macroscopic scale rather than a quantum scale. The region surrounding the NMR tool can be uniformly divided into a grid of volume elements, commonly termed xe2x80x9cvoxels,xe2x80x9d that can be referenced using a suitable coordinate system. One such convenient coordinate system is a cylindrical polar coordinate system. Each voxel contains many hundreds of thousands of nuclei, but each voxel is small in comparison with the dimensions of the sensor. Within each voxel, the static magnetic field can be represented by a vector B0 and the magnetization by a vector M, both with classical properties. In this way, the quantum nature of the NMR phenomenon may be conveniently set aside. Hereafter, the magnetization in the voxels is loosely referred to as xe2x80x9cspin.xe2x80x9d
In equilibrium conditions, the quantities B0 and M are related by the expression                     M        =                                                            N                A                            ⁢                              γ                2                            ⁢                              h                2                            ⁢                              I                ⁡                                  (                                      I                    +                    1                                    )                                                                    3              ⁢              kT                                ⁢                      B            0                                              (        1        )            
where NA is Avogadro""s number, xcex3 is the gyromagnetic ratio of the nucleus, h is Planck""s constant, I is the nuclear spin, k is Boltzmann""s constant, and T is the absolute temperature. Associated with the magnetic field strength |B0| is a characteristic frequency, called the Larmor frequency, given by
xe2x80x83xcfx890=xcex3|B0|xe2x80x83xe2x80x83(2)
The equilibrium condition can be disturbed by applying a pulse of an oscillating magnetic field, represented by B1; this is called a radio frequency or RF pulse. Spins that have Larmor frequency at or near the frequency of the applied oscillating magnetic field experience a torque, as described by the Larmor equation                                           ⅆ            M                                ⅆ            t                          =                  M          xc3x97          γ          ⁢                      xe2x80x83                    ⁢                      B            0                                              (        3        )            
where x denotes the vector cross product. This expression describes a resonant condition: spins with a Larmor frequency that matches the applied field frequency are tipped away from the static field direction by an angle (in radians) given by the equation
xcex8=xcex3|B1|tp/2xe2x80x83xe2x80x83(4)
where tp is the duration of the pulse.
Those spins xe2x80x9con resonance,xe2x80x9d i.e., having a Larmor frequency that exactly matches the applied oscillating field will precess around the static field at the Larmor frequency. At the same time, the spins return to the equilibrium direction, i.e., aligned with the static field, according to a characteristic decay time constant known as the xe2x80x9cspin-lattice relaxation timexe2x80x9d or xe2x80x9cT1.xe2x80x9d
For hydrogen nuclei, xcex3/2xcfx80=42.58 MHz/T, so that a static field of 0.0235 Tesla, would produce a precession frequency of 1 MHz. U.S. Pat. No. 4,933,638 discloses a wireline NMR logging tool that operates at a frequency of 1 MHz, which is typical of prior art tools. The decay constant T1 is controlled by the molecular environment and is typically ten to one thousand ms. in rocks.
At the end of a ninety degree tipping pulse, all the spins on resonance are pointed in a common direction perpendicular to the static field, and they all precess at the Larmor frequency. The precessing spins are detected by a voltage induced in a receiving coil. This may be the same coil as used to produce the B1 field or another suitably oriented coil. According to the principle of reciprocity the component of nuclear magnetization that is precessing in a plane perpendicular to the field that would be produced by current flowing in the receiving coil induces a voltage in the receiver coil that can be amplified and measured. The voltage appearing on the receiver coil is the summation of all signals from the precessing spin system in the region of interest. The decay of the precessing pulses gives useful information about the fluid content in the formation surrounding the borehole. In particular, the dominant contribution to the signal arises from the precession of hydrogen nucleii and are thus a good indicator of the amount of water and hydrocarbons in the formation.
The magnets and the RF coils are positioned so that the static and the RF fields are perpendicular to each other at least over a portion of the formation surrounding the NMR tool where the static field has a substantially uniform strength. This region is the region of interest or region of examination. The NMR measurements corresponding to such region are needed to determine the formation parameters of interest. At the field strengths typically used in NMR tools, the region of examination can overlap a part of the wellbore, which can severely affect the formation measurements due to the fluid in the wellbore.
A problem with prior art techniques is that the signal to noise ratio of the precessing signals is small. As noted above, the net magnetization is a function of the excess of those spins that are aligned parallel to the applied magnetic field and those spins that are aligned anti-parallel to the applied magnetic field. This excess is primarily a function of the strength of the applied magnetic field.
It would be desirable to have an NMR logging tool in which much higher magnetic fields are used than in prior art: this would facilitate having the region of examination further away from the borehole while, at the same time, increasing the signal to noise ratio of the NMR signals. The present invention satisfies this need.
The present invention discloses a method and apparatus for determining a characteristic of an earth formation surrounding a borehole in which a pulsed nuclear magnetic resonance (NMR) tool is received. A static magnetic field is produced in the borehole using Trapped Field Magnets (TFMs). The term TFM refers to a superconducting material below its critical temperature Tc having a circulating current therein, the current being able to flow indefinitely within the superconducting material, thereby sustaining a magnetic field. The TFMs are made of material having a high Tc, so that the magnetic field can be sustained for the duration of the well logging by enclosing the TFMs within a cryostat containing liquid nitrogen or liquid helium as a coolant, or using a cryocooler. The magnets are configured to provide a region of examination within the formation and at a distance form the borehole with the desired field strength. By using the TFMs, the field strength within this region is much higher than is attainable with conventional permanent magnets, giving a large signal to noise (SIN) ratio for the NMR signals. The magnetic field within the TFMs is kept at a low enough value that instability problems associated with these materials do not arise. This makes it possible to use the TFMs in an MWD environment. In one embodiment of the invention, the TFMs are magnetized outside the borehole environment using conventional high field strength electromagnets prior to emplacement within the cryostats. In another embodiment of the invention, vortex currents within the TFMs are induced in situ over a period of time, so that the power requirements for the inducing field are attainable in a borehole environment.
A radio frequency (RF) magnetic field is produced using a RF antenna in the NMR tool that is orthogonal to the direction of the static magnetic field. The RF magnetic field comprises a RF magnetic field modulated by a sequence of pulses. Such sequences of pulses are known in the art. An induced signal is received relating to a parameter of interest in the formations.